One of conventional optical filter devices includes an optical distributed feedback structure provided in an optical amplifying device which utilizes a semiconductor active layer. In the optical filter device, a variable tuning of a predetermined wavelength which is selected from wavelength division multiplexed optical signals can be performed by changing a carrier density injected into the active layer. Further, the optical filter device is of a transmission type which is well adapted to a higher integration. For these reasons, the optical filter device is expected to be applied to a wide practical use. Especially, a distributed feedback structure having a diffraction grating is more advantageous in regard to properties of the wavelength selection and integration for the optical feedback structure than a Fabry Perot resonator having cleaved facets. Such an optical filter device as using the distributed feedback structure has been proposed on pages 123 to 126 of "Optics Communications", Volume 10, No. 2, February 1974, and theoretical studies thereof have been made therein.
As a further conventional optical filter having no optical gain, an optical filter including a .lambda./4 shifted diffraction grating provided after the growth of an optical guide layer on a semiconductor substrate has been proposed. The .lambda./4 shifted diffraction grating is a diffraction grating in which a phase thereof is deviated in the vicinity of the center position by .lambda./4, where .lambda. is a wavelength of light transmitted therethrough. The feature of this optical filter is to provide a transmission wavelength band of a narrow width less than 1 to 2 .ANG. in the vicinity of Bragg wavelength. This type of an optical filter is described on pages 125 to 127 of "Applied Physics Letters", Vol. 49, 1986 by R. C. Alferness et al.
Such an optical filter device as the former and latter optical filter devices described above has a function in which a predetermined wavelength of an optical signal is selected from wavelength division multiplexed optical signals, so that the optical filter device plays an important role in such a wide use as in an optical communication system, optical switching apparatus, and optical signal processing, etc. Accordingly, it is strongly desired that the satisfactory selection of a wavelength and wide variable tuning range of a wavelength selection are obtained in the optical filter device. Further, it is indispensable that the optical filter device is of a structure of an optical integrated circuit. Therefore the optical filter device is required to be of a transmission type in which only a predetermined wavelength of an optical signal is transmitted therethrough.
However, the following disadvantages occur in optical filters previously proposed and studied in the art. First of all, the optical filter explained in the aforementioned "Optics Communications" article will be discussed. The optical filter comprises an optical guide layer for guiding light and an active layer for amplifying light, respectively, grown on a distributed feedback structure including a diffraction grating by crystal growth. A bandgap energy of the optical guide layer is larger than an energy of a transmission light wavelength, and a bandgap energy of the active layer is proximate to the transmission light wavelength energy. When current is injected into the optical filter, effective refractive index n.sub.o lowers due to the increase of carrier density. This is widely known as "plasma effect". The wavelength .lambda. of light which is selected in the optical filter is defined in the following equation. EQU .lambda.=2n.sub.o .LAMBDA.
where .LAMBDA. is a pitch of the diffraction grating. Therefore, the wavelength .lambda. to be selected is controlled to Vary by changing the effective refractive index n.sub.o in accordance with the current injection. When current is injected into the active layer for the tuning of the selected wavelength, optical gain of the selected wavelength is increased and the wavelength selectively becomes sharp. On the contrary, where the injected current is increased up to a level more than a certain current value, laser oscillation is induced in the device. That is, the device is no longer an optical filter, but is a distributed feedback laser. In order that the device is used for an optical filter, injected current must be less than a threshold current for laser oscillation, and concurrently must be more than a predetermined current by an amount which is enough that a desirable wavelength selectivity is attained. Accordingly, a current range to be supplied to the device is limited, resulting in a limited tuning range. Consequently, the optical filter is only applied to an optical communication system having several channels.
On the other hand, in the optical filter described in the "Applied Physics Letters" article by Alferness et al., explained above, it is impossible to vary transmission light wavelength.